1. Field of the Invention
The present invention relates generally to the repair of multi-chip modules and, more particularly, to a process and apparatus for removing one or more chips from a mult-chip module in which the chips are so closely mounted to each other on the module the chips are described as being brickwalled.
2. Description of the Prior Art
To increase the number of flip chips being C4 joined to a multi-chip module and thus increase the density without increasing the physical size of the module, the chip are being placed so close together that the sides of the chips are nearly touching each other. The removal of one or more chips is required due a defective chip or C4 joint, discovered during testing, or to upgrade the module function by replacing one or more of the mounted chips with one or more higher performing chips. One present method and apparatus for removing chips from a module employ gripping clips which are used in conjunction with a bi-metallic disc. One of the clips physically contacts one side of a chip to be removed and the other clip contacts the opposite side of the chip. When the module is heated to a temperature to cause the C4 joints to become molten, a bi-metallic disc provides an upward tensile force on the gripping clips to remove the gripped chip from the module. Examples of chip removal by gripping the opposite sides of a chip is found in U.S. Pat. Nos. 4,485,958, 5,553,766 and 5,779,133, and IBM TDB Vol 18, No 9, February 1976, pp 2850.
However, because of the physical size of the gripping clips, a minimum spacing of 2 mm is required between chips in order to remove a chip with such gripping clips. This minimum spacing requirement wastes space on the top surface of the module and prevents closely spaced mounting of chips previously described as brickwalling.
Other processes and apparatuses for removing flip chips from a module without gripping the opposite sides of a chip are hot vacuum as described in U.S. Pat. No. 5,605,277 and hot gas as disclosed in U.S. Pat. No. 5,598,965. Neither of these processes and apparatus are suitable for brickwall mounted chips because of a large spacing is required between chips so that the chip to be removed can be isolated from the other chips.
For removing non-flip chips or chips without C4 joints in which the back of the chips are mounted to the module, a process and apparatus is described in U.S. Pat. No. 5,154,793 which permits brickwall mounting but, because this process and apparatus is used for removing back bonded non-flip chips, the temperature required is not sufficient high to melt C4 joints for the removal of flip chips in the temperature range of about 340 to about 375xc2x0 C.
Consequently, there is a need for a flip chip removal process and apparatus which is able to remove chips from brickwall mounted chips without damaging the removed chips or chips adjacent to the removed chips. Therefore, it is an object of the present invention to provide a low cost, highly reliable process and apparatus which does not require grippers to remove the flip chips and which permit minimal spacing between chips to achieve brickwalling. A further object of the present invention is to provide a process and apparatus which pulls the flip chip from the back surface of the chip thereby not requiring any spacing between the chips. A final object of the present invention is provide a process and apparatus in which the conventional furnace for C4 joining can be used for removing selected flip chips while simultaneously joining additional chips on other locations of the multi-chip module.
To achieve the foregoing objects, the present invention comprises the process of bonding a removal member, of a dimension smaller than the area of the chip, to the exposed back of the chip with a low temperature curable thermal adhesive which, when cured is able to withstand the high temperatures capable of melting the C4 joints. A tensile force is applied to the removal member which is greater than the surface tension of molten C4 joints of the chip or chips to be removed. The mult-chip module is then heated in an ambient controlled furnace to a temperature to melt the C4 joints. The tensile force necessary will vary depending upon the number of C4 joints. The larger the number of C4 joints, the greater the tensile force required to overcome the surface tension of the C4 joints. Due the tensile force applied, either before or during the heating of the module, to the removal member on the backs of the chip or chips to be removed, the chip or chips will be lifted from the module when the C4 joints become molten. After cooling, the lifted chip or chips are removed from the module while the C4 joints of the remaining chips are solidified to the pads on the module.
Another aspect of the present invention is an apparatus for removing brickwalled chips comprising a removing assembly including a removal member, bonding means for attaching the removal member to the back of a chip, and tension means to apply a minimal tensile force to the removal member and the back of the chip to be removed either prior to heating the module to a temperature to melt the C4 joints or, alternatively, during such heating.
A further aspect of the present invention is the process of removing a chip from a multi-chip module comprising the steps of bonding one end of a removal member to the back of the chip to be removed, followed by applying a tension force to the other end of the removal member, before or during heating of the module to a temperature to cause the C4 joints to melt, which is greater than the surface tension of melted C4 joints; and then heating the module to that melting temperature to separate the chip to be removed from the module.